1. Field of the Invention
This invention relates to conductive polymer compositions and strip heaters comprising them, in particular self-regulating strip heaters which comprise a pair of elongate metal electrodes embedded in an elongate core of a conductive polymer composition which exhibits PTC behavior.
2. Introduction to the Invention
Conductive polymer compositions and self-regulating strip heaters which comprise conductive polymer compositions are well known. A conductive polymer composition comprises a polymeric component and, dispersed or otherwise distributed therein, a particulate conductive filler. For most applications, such strip heaters comprise a resistive element composed of a conductive polymer having elongate electrodes embedded therein. Generally, the resistive element is surrounded by an insulating jacket to provide electrical insulation and environmental protection. In operation, these heaters can be wrapped around or attached to a substrate, e.g. a pipe or a tank, and provide a varying level of heat in response to changes in the thermal environment. Under normal operating conditions, this self-regulating feature serves to limit the maximum temperature which the heater achieves, thus providing safety and reliability. However, where the electrodes are exposed by external damage or by faulty installation, and when the heater is electrically powered and exposed to an electrolyte, in some circumstances an arc can occur between the electrodes. If the heater remains powered, the arc can under some circumstances "propagate", i.e. progress down the length of the strip, prolonging the burning.
Various solutions to this problem have been proposed, including the use of polymers which are themselves flame-retarded and the use of conductive polymer compositions which comprise flame-retardant additives, and the use of circuit protection devices such as arc fault interrupters or ground fault interrupters which remove power from the circuit in the event of an arc. Copending, commonly assigned application Ser. No. 07/519,701 (Batliwalla et al), filed May 7, 1990, now abandoned in favor of a continuation-in-part application, application Ser. No. 08/211,829, filed Nov. 6, 1992, the disclosure of which is incorporated herein by reference, discloses the use of an additional insulating jacket over the resistive element in order to reduce the flammability of the heater.